Ultrasonic diagnostic devices are one of common image display devices in clinical practice together with CTs and MRIs, and have features of compact in size and enabling motion picture imaging. Although the viewing field is limited, ultrasonic diagnostic devices have excellent spatial and time resolutions, and with the use of an ultrasonic contrast agent, micro-structured blood flow dynamics can be observed, and so the ultrasonic diagnostic devices have an important function to diagnose properties of tumors.
Although CTA (CT Angiography) has been a main-stream for observation means of blood flow dynamics, CTA has problems of exposure to radiation and toxicity of iodine contrast agents. Therefore the usage of CTA may be limited for patients suffering from kidney disease or having an allergy. Compared with this, ultrasonic contrast agents are micro-bubbles of a few μm in diameter and the agents themselves do not have any toxicity. Further since ultrasonic contrast agents have properties of resonating in a frequency bandwidth of medical ultrasonic waves to emit intense radiation of harmonic components, having a size equal to red blood cells to enter into small vessels, and being discharged from the body with the course of time mainly through exchange of gases in the lung, blood flow dynamics of small vessels can be observed easily.
Liver tumors are one of lesions that can be effectively observed with blood flow dynamics. It is known about liver tumors that blood vessels that are dominant at the lesion part shift from portal to asteriosity during the course of worsening from precancer states such as hepatitis and cirrhosis to liver cancer. It is known that as for liver tumors at an early stage, portal veins are dominant, through which blood via a digestive system flows, and as the clinical state progresses, dominant vessels shift to arteries (Non-Patent Literature 1).
Arteries and portal veins are different in inflow starting time and inflow-speed of a contrast agent due to their different circulatory pathways. Effective means to evaluate such a difference in blood flow dynamics is TIC (Time-Intensity Curve) that is obtained by plotting a change of brightness over time due to the inflow of a contrast agent. A contrast-enhanced image or the TIC of a lesion part enables early detection of the lesion or determination of its activity. Furthermore, since blood flow dynamics vary with types of lesions such as hepatocarcinoma, metastatic cancer and cyst, the contrast-enhanced ultrasonic techniques are regarded as important for effective imaging techniques in differential diagnosis of lesions.
The evaluation using blood flow dynamics is effective also to determine the effectiveness of cancer therapy. In the case of medical therapy using RF or medicines, blood flow dynamics in addition to the size of tumors are important targets of observation. This is because, even when a tumor does not change in size on the image, the effectiveness of the therapy can be determined based on dissipation of tumor vessels or a decrease in the amount of the blood flow Especially in the case of therapy such as an anti-angiogenic agent or vascular embolization therapy targeting at vessels serving as nutrient supply for tumors, the future therapeutic strategy will be decided by not only the presence or not of blood flow into a lesion but also whether the blood flow found is portal or asteriosity, and therefore blood flow dynamics are important targets of the observation to determine the effectiveness.
The technique described in Patent Literature 1 is to calculate statistic values such as average brightness values based on the measured TIC and display a color-coded image in accordance with the values. In this technique, firstly tissue is filled with a contrast agent under the irradiation of ultrasonic waves at a medium to low sound pressure level. Subsequently the contrast agent is cleaned out by irradiation of ultrasonic waves at a high sound pressure level, and a blood flow flowing back to the same imaging plane under irradiation of ultrasonic waves at a medium to low sound pressure level again is evaluated with TIC.
The technique described in Patent Literature 2 is to detect the inflow time of a contrast agent for each vessel and display relative differences therebetween in different colors. A region of interest is provided on a screen, and an image is colored while setting the inflow time in this region as a reference time.